The present invention relates to an improvement in a data reader which compares an analog signal from a reading means for reading recorded data with a reference voltage signal so as to convert the analog signal into a digital signal.
An optical data reader such as a bar code reader is conventionally known. In such a reader, light is radiated onto a medium on which predetermined data is recorded (printed) in the form of bar codes. The differences in reflectivity are converted into an electrical signal (magnitude of a current) for allowing reading of the bar code data. A data reader for magnetically reading such data is also known.
Methods of converting an analog signal into a digital signal of logic level "0" or "1" include the DC method and the AC method. FIG. 1 shows a conventional bar code reader which adopts the DC method. The bar code reader has a hand scanner 2 for manually scanning a medium (label) 1 on which bar codes are recorded, an amplifier 3 for amplifying the analog signal read by the hand scanner 2, a comparator 4 which receives at its positive input terminal an analog signal from the amplifier 3, and a variable resistor 5 for supplying a reference input voltage to the negative input terminal of the comparator 4. The comparator 4 produces a digital signal which corresponds to the output voltage from the amplifier 3.
A conventional bar code reader adopting the AC method has, for example, the structure as shown in FIG. 2. The same reference numerals as used in FIG. 1 denote the same parts as in FIG. 1, and a detailed description thereof will be omitted. The bar code reader further has a differentiator 6 consisting of a capacitor C1 and a resistor R. The differentiator 6 receives an output voltage from the amplifier 3 and cuts off the DC component of the input voltage so as to obtain an AC component thereof. The reader also has a buffer amplifier 7 for non-inverting amplification of an output from the differentiator 6. An output from the buffer amplifier 7 is supplied to the positive input terminal of the comparator 4. In this case, the negative input terminal of the comparator 4 receives a ground level voltage as a reference voltage.
In each of the bar code readers adopting the AC and DC methods, the reflectivity differs depending upon the quality of the label 1 or the quality of the ink used for printing the black bars. Then, the waveform of an output signal from the amplifier 3 has a different amplitude and includes a DC component. If the densely arranged bars are thin, a DC component is further included in the output signal from the amplifier 3 which has a waveform as shown in FIG. 3 waveform (A).
In view of this problem, the reference voltage to be supplied to the comparator 4 must be adjusted by the variable resistor 5 in a bar code reader adopting the DC method. However, such adjustment is extremely difficult to perform. On the other hand, in the reader adopting the AC method, an output from the buffer amplifier 7 has an attenuated oscillating waveform which is obtained by non-inverting amplification of an AC component after cutting off a DC component, by means of the differentiator 6, as shown in FIG. 3 waveform (B). When an output from the buffer amplifier 7 is compared with ground level (GND) by the comparator 4, the input signal does not fall below the threshold level at its initial portion, as shown in FIG. 3 waveform (C), so that correct A/D conversion cannot be performed. As a result, a bar code reader of this type cannot read densely recorded bars and can only read loosely recorded bar codes.
Since the attenuated oscillating waveform is compared with a predetermined threshold level, the pulse width of the resultant digital signal is significantly disturbed.